Method for the preparation of stable aqueous dispersions of polymer beads and the use of these dispersions in photographic elements

ABSTRACT

Method for the preparation of stable aqueous dispersions of finely divided solid spherical polymer beads having an average particle size between about 0.5 and about 5 μm and having a glass transition temperature of at least 40° C. by dissolving in an aqueous solvent mixture at least one α,β-ethylenically unsaturated monomer capable of forming a polymer that is soluble in the monomer(s) present in said aqueous solvent mixture but which is insoluble in said aqueous solvent mixture, a free radical-forming polymerization initiator, and a graft-polymerizable polymer containing hydrophilic groups, heating the solution to a temperature from 50° C. to the reflux temperature thereof with stirring so as to form said polymer beads. The polymer beads can be used in an antifriction surface layer, an antihalation surface layer, an antistatic surface layer, or in a protective surface layer of a photographic element or in a top layer of a drafting film containing a photosensitive layer.

This is a continuation of application Ser. No. 06,442,021 filed Nov. 16,1982, now abandoned.

The present invention relates to a method for the preparation of stableaqueous dispersions of finely divided solid polymer beads and to the useof those dispersions in photographic elements.

It is generally known to protect photographic elements againstmechanical damage or undesired adverse influences on their photographiccharacteristics by coating them with thin protective surface layers,usually thin hardened gelatin layers.

Mechanical damage of photographic elements or adverse influencing of thephotographic characteristics thereof are often incurred indeed e.g. whena layer element is conveyed in dry state over or between rollers andcontacting surfaces. Protective gelatin surface layers, however, tend toslide with relative difficulty on surfaces with which they enter intomoving contact.

It is also known in the photographic art to use light-sensitive silverhalide elements containing matting agents e.g. inorganic pigments suchas finely divided silica particles in a surface layer. Silica particlescan be incorporated as matting agents into the surface layer(s) ofphotographic elements for different purposes. For instance, it has beenproposed to include finely divided silica particles in the surfacelayer(s) of photographic elements so as to reduce the sticking tendencyof said element, e.g. when stored or packed in rolls or stacks. Theroughness resulting from the silica particles at the surface of thephotographic element gives a matted appearance to the material and alsoprevents the generation of static electricity, which may cause sparksand thus exposure of the photographic light-sensitive layer(s), as wellas the formation of Newton's ring during printing and enlargingoperations, thanks to the fact that the contact surface of said elementagainst another object has become relatively small. Furthermore, thesurface layers comprising silica particles may also reduce abrasion bydry-friction and diminish the scratchability of said photographicelements that are stored or packed in contact with other materials, asis the case e.g. with X-ray material packed without interleaves(non-interleaved fold-scratching).

However, the use of silica and other matting particles in a surfacelayer, e.g. a protective layer applied to the light-sensitive silverhalide emulsion layer of a photographic element, also presents a numberof drawbacks. For instance it has been observed that the decrease ofabrasion is often insufficient as a result of the rough irregularsurface of the larger part of the known matting agents. Also, thepresence of matting particles in a surface layer of a photographicelement may produce an objectionable milky appearance, which--howeverweak it may be--is undesirable especially when transparent images haveto be obtained. As a matter of course transparent images should beentirely free from milky haze and yet the photographic elements leadingto those images need protection against mechanical damage or adverseinfluences on their photographic characteristics as described above.Furthermore a relatively high density of matting particles e.g. silicaparticles is necessary, which may cause them not to remain suspended inthe coating composition. The refractive index may differ too much fromthat of gelatin, which may cause the silver image to look less black.

It has also been proposed to use finely divided cellulose or derivativesthereof as matting agent. In that case it is essential that thecellulose or derivative thereof be comminuted mechanically or chemicallyto the required grain size, which involves tedious additional measures.

Hydrophobic polymers may be dissolved in an organic solvent and thissolution dispersed in water containing a hydrophilic colloid such asgelatin. Upon evaporation of the organic solvent interesting dispersionscan be formed having a particle size as required for matting agents. Thedisadvantage of this process is the almost unavoidable presence ofresidual amounts of organic solvent, which gives rise to undesiredconglomeration of the polymer particles.

It is further known that very fine dispersions of polymeric materialsare obtained when the polymers are formed according to an emulsionpolymerisation process. The particle size of emulsion polymer particles,however, is always less than 0.5 μm, and on the average less than 0.1μm, which makes them inappropriate for use as matting agents.

As a result thereof the particle size of the majority of the mattingagents so far proposed is either too small or their grain is too coarseand consequently an undesirable clouding forms at the surface of thephotographic layer, to which they are applied.

In U.S. Pat. No. 3,941,727 it has been proposed to use polymer particlessizing between 1 and 10 μm as matting agents in hydrophilic coatingcompositions. Yet, the preparation of these polymer particles and thecontrol of their size is cumbersome and requires the use of complexapparatuses. Moreover, the size distribution of the particles producedis far too wide so that regretfully disturbing amounts of undersizedparticles are present as well. Furthermore it is also difficult toprepare polymer particles with a predetermined average particle size.

It is an object of the present invention to provide a method forpreparing stable aqueous dispersions of solid, non-abrasive, sphericalpolymer beads having an average particle size between 0.5 and 5 μm andbeing substantially homodisperse i.e. having a substantially uniformsize frequency distribution.

A further object is to prepare said dispersions of polymer beads by asimple preparative method in a one-step reaction procedure.

A further object is to provide a method for the preparation of saiddispersion of polymer beads with a predetermined average particle sizewithin the above range.

A still further object is to provide a photographic silver halideelement comprising in at least one silver halide emulsion layer or otherlayer and/or in the support polymer beads obtained by said method.

Another object is to provide a photographic element comprising in (a)surface layer(s) said polymer beads protruding from said surfacelayer(s) and thus acting as spacing agents that provide a safe distancebetween said surface layer(s) and contacting objects so as to protectthe surface(s) of said photographic element against mechanical,physical, or chemical influences e.g. dry friction of said photographicelement against contacting objects and transfer or diffusion of matterbetween said photographic element and contacting objects.

Contacting objects are e.g. rollers or guide members, used inapparatuses for the manufacture, packaging, exposure etc. ofphotographic elements. Contacting objects can also be photographicelements themselves. For instance, when the photographic element iswound on a reel or stacked in piles, the backing layer of said elementis in contact with the uppermost layer of said element.

Other objects of the invention will become apparent from the descriptionhereinafter.

The above objects are accomplished by a method for the preparation ofstable aqueous dispersions of finely divided solid spherical polymerbeads having an average particle size between about 0.5 and about 5 μmand having a glass transition temperature of at least 40° C., comprisingthe steps of:

(A) dissolving in an aqueous solvent mixture of water and at least onewater-miscible polar organic solvent

(1) at least one α, β-ethylenically unsaturated monomer capable offorming a polymer that is soluble in the monomer(s) present in saidaqueous solvent mixture but which is insoluble in said aqueous solventmixture,

(2) a free radical-forming polymerization initiator (e.g. potassium,sodium, or ammonium persulphate) that is soluble in the aqueous solventmixture, and

(3) a graft-polymerizable polymer containing hydrophilic groups (e.g.sodium or potassium carboxylate or sulphonate groups, hydroxide groups,ethylene oxide groups, and amide or cyclic amide groups), and capable offorming a graft polymer that remains soluble in said aqueous solventmixture,

the weight ratio of said graft-polymerizable polymer to said monomer(s)being in the range from 1.5:100 to 8:100 and the weight ratio ofpolymerization initiator to monomer(s) from 0.1:100 to 5:100, and

(B) heating the solution obtained to a temperature from 50° C. to thereflux temperature thereof, with continuous stirring to initiate bypolymerization the simultaneous massive formation of homopolymer orcopolymer from said monomer(s) and precipitation thereof, and theformation of a small proportion of graft polymer.

The graft polymer formed and incorporated in the product beadsstabilizes the homopolymer or copolymer, which forms the majorproportion of the beads, as hereinafter described.

The graft-polymerizable polymer used is a homopolymer or copolymer,which in the presence of radicals and in the conditions described abovefor the preparation of the polymer beads can be converted into a livingmolecule, on which by graft-copolymerization side-chains can beimplanted. The formation of the living molecule can occur by withdrawalof a labile hydrogen atom or by conversion of originally implantedunsaturated hydrocarbon groups e.g. acrylate groups in the (co)polymer.

Before the beginning of the polymerization the reaction medium mainlyconsists of a homogeneous solution at room temperature, in the solventmixture, of the graft-polymerizable polymer, the water-soluble freeradical-forming polymerization initiator, and at least oneα,β-ethylenically unsaturated monomer.

By heating this reaction medium the dissolved initiator decomposes andforms free radicals, which then enter into reaction with the dissolvedgraft-polymerizable polymer either via a labile hydrogen atom or via areactive position and thus form living molecules, which while remainingdissolved in the aqueous solvent mixture, encounter either reactivemonomers or already growing polymer chains of such monomers, thusforming a graft polymer with the original graft-polymerizable polymer.

Two reactions are in fact taking place simultaneously, the firstreaction being the polymerization of the greater part of theα,β-unsaturated monomer(s) to form the polymer core of the final beadand the second reaction being the formation of polymer chains from avery small part of the α,β-unsaturated monomer(s), which chains graftonto the activated initial polymer.

Polymer beads are thus formed, which are composed of a nucleus and anenvelope.

The nucleus of the beads consists of a bundle of intertwisted polymerchains obtained by said polymerization (the above-mentioned firstreaction) of the monomer(s) and which is insoluble in the aqueoussolvent mixture, and of a small proportion of same polymer chainsobtained by copolymerization of the monomer(s) and the initial polymer(the above-mentioned second reaction), said same polymer chains beinginterwisted with the above-mentioned polymer chains but grafting by oneend on the envelope. For clarity's sake the second group of polymerchains will be named hereinafter "grafting polymer chains".

The envelope of the beads mainly consists of the initialgraft-polymerizable polymer, which after the above-mentioned secondreaction layer-wise surrounds the nucleus and carries said graftingpolymer chains, which stretch into the nucleus and make part thereof.The enveloping polymer acts as stabilizer for the nucleus of the beads.

The most important characteristic of the dispersions according to theinvention is that discrete solid polymer beads are formed, which arestabilized sterically as a result of the stable circular arrangement inspace of the atoms of the stabilizer around the nucleus. Moreover, whenthe stabilizer comprises ionic groups, these groups while remainingattached to the stabilizer extend radially from the bead surface intothe aqueous solvent medium and form a solvate therewith. This solvationhas a stabilizing effect that is supplemental to the stericstabilization.

As a result of the anchoring of the envelope to the nucleus and theabsence of enclosed solvent in the beads, the formation of conglomeratesof beads upon dilution of the dispersion e.g. mixing of the dispersionwith the coating composition for a photographic layer, is prevented.

Another advantage of the invention is that with conventional reactionapparatuses polymer beads can be prepared, the average size of whichremains within a very narrow range that can be chosen from about 0.5 toabout 5 μm.

A further advantage of the invention is the total absence of any polymerfraction in latex form, in other words of under-sized polymer particleshaving a diameter of 0.1 μm or less. This is to be contrasted withpreviously known methods of bead preparations, which occur in thepresence of large amounts of hydrophilic protective colloids such ase.g. gelatin, polyvinyl alcohol, poly-N-vinyl pyrrolidone, orwater-soluble cellulose derivatives.

The solid polymer beads prepared according to the method of the presentinvention have the following characteristics:

they are readily dispersible in water or in an aqueous solvent mixturewithout formation of conglomerates

they are substantially miscible with aqueous colloid solutions such ase.g. solutions of gelatin, polyvinyl alcohol, dextran, poly-N-vinylpyrrolidone, and water-soluble cellulose derivatives without formationof conglomerates

they have a regular spherical shape

they have an average size between 0.5 and 5 μm

they have a narrow size distribution

they have a glass transition temperature of at least 40° C. and thus arehighly resistant against mechanical deformation.

Any polar organic liquid that is substantially miscible with water canbe used as inert water-miscible solvent for forming together with waterthe aqueous solvent mixture. Mixtures of several polar organic liquidscan be used also together with water to form the aqueous solventmixture.

Suitable polar organic liquids that are substantially miscible withwater and that are solvents for the monomer(s) added are the loweralcohols e.g. methanol, ethanol, and isopropanol and dioxan, acetone,acetonitrile, dimethylformamide, etc.

The organic solvent(s) and the proportion thereof to the water presentin the aqueous solvent mixture are chosen such that prior to thepolymerization the aqueous solvent mixture is a solvent for thegraft-polymerizable polymer containing hydrophilic groups, for theα,β-ethylenically unsaturated monomer(s), and for the initiator, andthat after the polymerization it is a non-solvent for the homopolymer orcopolymer obtained from the monomer(s) but remains a solvent for thegraft polymer formed.

It is possible to influence the results as to the nature and size of thepolymer beads into a desired sense by changing the quantitativeproportion of organic solvent(s) to water.

The optimum quantitative proportion between these solvents can easily bedetermined for any selected combination of graft-polymerizable polymerand monomer(s) by making a few tests with changing amounts of organicsolvent and water. For instance in the combination of co(styrene/maleicacid monosodium salt) as graft-polymerizable polymer, methylmethacrylate as monomer, and potassium persulphate as initiator thedesired average bead size obtained can be predetermined by selecting agiven quantitative proportion of water and water-miscible solvent e.g.ethanol. A quantitative proportion of 40 parts by volume of water and 60parts by volume of ethanol yields a mixture of very coarse beads and alarge amount of amorphous precipitate owing to insufficient solubilityof the graft polymer formed. A proportion of water/ethanol (43/57)yields beads with an average size of approximately 4 μm. A proportion of46/54 gives beads sizing 2.79 μm; 50/50 gives beads of 2.56 μm, and60/40 produces beads of 0.98 μm. A higher proportion of water results inthe formation of heterodisperse beads with a very large fraction of verysmall particles owing to insufficient solubility of theα,β-ethylenically unsaturated monomer(s).

When styrene is used as monomer instead of methyl methacrylate, goodresults are obtained with a solvent mixture of 30 percent by volume ofwater and 70 percent by volume of ethanol, to provide homodisperse beadswith an average size of 2 μm.

The monomer(s) used in the method of the present invention are chosen sothat they are soluble in the aqueous solvent mixture. The polymer(s)formed therefrom are insoluble in the aqueous solvent mixture butsoluble in the monomer(s) used, and the polymer beads have a glasstransition temperature (Tg) of at least 40° C.

Suitable α,β-ethylenically unsaturated monomers for use in thepreparation of the polymer beads are e.g. styrene, vinyltoluene andsubstituted vinyltoluene e.g. vinyl benzyl chloride and the homologuesthereof, chlorostyrene, alkyl methacrylates e.g. methyl methacrylate,ethyl methacrylate, propyl methacrylate, butyl methacrylate and thehigher methacrylates, e.g. stearyl methacrylate; substituted alkylmethacrylates e.g. hydroxyethyl methacrylate; butadiene, isobutylene,chlorobutadiene, 2-methylbutadiene; vinyl pyridines e.g. 2- and4-vinylpyridine, etc. A combination of these monomers as well as one ofthem alone may be chosen depending on the particular needs. Othermonomers than those listed above can be used if only they fulfil thesolubility and Tg requirements set. It is possible to combine one ormore of the monomers described above with other monomers that themselvesdo not comply with the requirements described herein for theα,β-ethylenically unsaturated monomers. For instance vinylidenechloride, vinyl chloride, acrylonitrile, and methacrylonitrile are notsolvents for their own polymers and can thus not be used for theformation of homopolymers. Nevertheless they can be combined with one ormore suitable monomer complying with the requirements set forth to formcopolymers that are soluble in the latter monomers.

The graft-polymerizable polymers should:

be sufficiently reactive to form radical-graft-copolymers with theα,β-ethylenically unsaturated monomer(s) present

contain hydrophilic groups such as hydroxide, oxide, amide, orcarboxylic acid and sulphonic acid groups, which may be neutralizedcompletely or partially with potassium or sodium hydroxide,

contain these hydrophilic groups in a number sufficient to make thepolymer beads to be formed, stable in aqueous medium,

be soluble in the reaction medium of aqueous solvent and unsaturatedmonomer(s).

Suitable graft-polymerizable polymers for use in the preparation ofpolymer beads are e.g. polyethylene oxide, low molecular weightpolyvinyl alcohol, polyvinyl pyrrolidone, co(vinyl alcohol/vinylacetate) containing 12 mol % of vinyl acetate units and the samecopolymer containing 40 mol % of vinyl acetate units, sodium orpotassium salts of co(acrylic acid/styrene) containing 40 to 60 mol % ofacrylic acid, co(vinyl acetate/crotonic acid), the reaction products ofcopoly(styrene/maleic anhydride), of copoly(vinyl acetate/maleicanhydride), of copoly(ethylene/maleic anhydride), or of copoly(N-vinylpyrrolidone/maleic anhydride) with hydroxyalkyl oraminoalkyl(meth)acrylates, co(styrene/maleic acid monosodium salt), andespecially the latter copolymer containing 50 mol% of styrene and 50mol% of maleic acid monosodium salt. Other graft-polymerizable polymerscan be used, which comply with the requirements hereinbefore set forth.

In the production of an aqueous homogeneous dispersion of polymer beadsfor use in a photographic element in accordance with the invention thesize of the beads is determined by the nature of the graft-polymerizablepolymer, but can also be controlled by adjustment of other reactionparameters e.g. the concentration of the α,β-ethylenically unsaturatedmonomer(s) and especially the proportion between the volumes of waterand of the water-miscible solvent in the aqueous solvent mixture.Polymer beads with a predetermined average size of e.g. 1 μm, 1.5 μm, 2μm, 3 μm, 4 μm, and 5 μm can be prepared in this way.

When in the making of the polymer beads co(styrene/maleic acidmonosodium salt) is used, which is composed of equimolar amounts of itsmonomer components, highly homodisperse beads having an average particlesize from 1.0 to 5 μm can be obtained. Other proportions of the monomercomponents can be used, of course, and may also lead to interestingresults.

In the method of the present invention the weight ratio of thegraft-polymerizable polymer to said monomer(s) is generally comprisedbetween 1.5:100 and 8:100. Optimum proportions for obtaining a givenaverage particle size can be easily determined by making some simpletests.

In the method of the present invention excellent results are obtainedwith the above co(styrene/maleic acid monosodium salt) in the presenceof methyl methacrylate monomer in an aqueous solvent mixture of equalvolumes of water and ethanol. A high yield of homodisperse polymer beadshaving a grain size of 2.2 μm and presenting a favourable filtrabilityand stability upon prolonged storage at room temperature without thecompulsive presence of a large amount of hydrophilic protective colloidsis obtained with a weight proportion of 5.0 g of the copolymer to 100 gof methyl methacrylate monomer.

When the weight of said copolymer in respect of said monomer is lowerede.g. to 2.6 g of said copolymer instead of 5.0 g, the polymer beads whenleft standing form a hard tough polymer mass, which, however, can beredispersed completely by stirring.

If the weight of said copolymer is even further lowered e.g. to 1.3 gper 100 g of methyl methacrylate, insufficiently stabilized, coarsepolymer particles are obtained, which instead of being spherical haveassumed an irregular eliptical shape and size from 10 to 50 μm.Moreover, a large proportion of amorphous precipitate is formed at thesame time, which strongly hinders isolation by filtration.

Further reduction of the amount of said copolymer results in an almostintegral precipitation of polymerized methyl methacrylate in the form ofa coarse amorphous polymer mass.

An increase, however, of the amount of said copolymer e.g. up to 7.5 gin respect of 100 g of methyl methacrylate monomer leads to theformation of polymer beads with reduced average size. Moreover, thereaction medium becomes more viscous thus rendering the isolation of thepolymer beads more difficult. The precipitation of amorphous polymermass also increases. A further increase e.g. to 10.0 g of said copolymerper 100 g of methyl methacrylate promotes the solubility and leads tothe formation of a shapeless polymer mass.

It has also been observed that in the preparation of beads withco(styrene/maleic acid monosodium salt) as graft-polymerizable polymerthe stability of the dispersion is connected with the pH-value as well.With methyl methacrylate as the α,β-ethylenically unsaturated monomerand co(styrene/maleic acid monosodium salt) as the graft-polymerizablepolymer the beads obtained are converted into conglomerates when the pHof the dispersion is lowered to 4.0 by means of hydrochloric acid as aresult of the conversion of the carboxylate groups of the polymer intoinsolubilizing free carboxylic acid groups. If, however, the pH-value isincreased again by means of sodium hydroxide, a stable dispersionwithout conglomerates is restored.

The polymerization initiator being soluble in the aqueous solventmixture and forming free radicals upon heating is generally present inan amount from 0.1 to 5% by weight based on the amount of monomer(s)present. Suitable polymerization initiators for use in the preparationof the polymer beads according to the invention are persulphates, e.g.potassium sodium and ammonium persulphates or mixtures thereof.

In the case of potassium persulphate amounts of 10.0×10⁻³ to 40×10⁻³ molof persulphate per liter of reaction medium yield excellent dispersionsof polymer beads. These polymer beads are particularly suitable for usein the surface layer(s) of a photographic element, in accordance withone aspect of the invention.

A reduction in the amount of persulphate as polymerization initiatorleads to the formation of larger polymer beads, whereas an increase inthe amount of persulphate entails a reduction in the size of the polymerbeads. As a consequence, the amount of persulphate in the reactionmedium constitutes a parameter that also defines the size of the polymerbeads. In other words the results aimed at can be attained bycontrolling i.a. the exact amount of the polymerization initiator.

It is possible to use the polymerization initiator in amounts outsidethe range given hereinbefore, though from 40×10⁻³ mol on of persulphateper liter of reaction medium the polymer beads seem too small and thusless apt for use in photographic layers and especially for antihalationsurface layers. Moreover, the pH of the reaction mixture falls owing tothe higher concentration of persulphate thus causing the polymer beadsto conglomerate in the case of graft-polymerizable polymers havingcarboxylic acid groups as hydrophilic groups. This can only be remediedby the addition of sodium hydroxide up to a pH of at least 5. Very lowamounts of 1.0×10⁻³ mol of persulphate fail to produce dispersions, butmainly form an amorphous precipitate.

When the above described combination of solvents, monomer, polymer, andpolymerization initiator are heated to the decomposition temperature ofthe initiator with thorough stirring, the radicals formed induce thepolymerization of the monomer(s) present. By doing so, graft-copolymersof the polymer present are formed as described hereinbefore.

The fact that graft-copolymers are formed indeed can readily bedemonstrated by a combination of titration and mass spectrometricanalyses. In order to illustrate this, the polymethyl methacrylate beadsstabilized with a graft copolymer of methyl methacrylate andco(styrene/maleic acid monosodium salt) as described hereinafter inPreparation 1, were examined as follows.

The bead fraction was isolated from the bead dispersion by centrifugingand divided in two parts. The first part was rinsed twice with theaqueous solvent mixture to form deposit D₁, and the second part of thebead fraction was carefully purified by dissolving in acetone, which isa non-solvent for the graft-polymerizable polymer co(styrene/maleic acidmonosodium salt), filtering the resulting acetonic solution, and pouringinto water, which had been acidified with hydrochloric acid, so as toform a sodium-free deposit D₂.

Both deposits D₁ and D₂ appeared to be fully identical according toinfrared examination. Their spectra substantially coincided with that ofmethyl methacrylate. However, a combined examination by pyrolysis andmass spectrometry clearly demonstrated in both D₁ and D₂ the presence ofstyrene in an order of magnitude smaller than 1.0% by weight.

After the isolation of the bead fraction from the bead dispersion bycentrifuging as described above, the dissolved polymer fraction wasisolated also from the remaining liquid phase by pouring the latter intowater, which had been acidified previously with an excess ofhydrochloric acid. The precipitating polymer, which appears to be freeof potassium as proven by pyrolysis on a platinum plate, was filteredoff and analysed also.

Infrared examination clearly showed the characterizing absorption bandsof styrene, of maleic acid, and of maleic anhydride. In consequence,however, of the masking effect of the absorption bands of the styrene,the maleic acid, and the maleic anhydride components the presence ofmethyl methacrylate was not demonstrable with certainty.

Yet, by application of the same combination of pyrolysis and massspectrometry as well as by functional titration with potassium methylateit could be established that the copolymer obtained from the liquidphase was not co(styrene/maleic acid) but manifestly a copolymer ofstyrene/maleic acid and methyl methacrylate in a proportion by weight of31.0/34.6/34.4.

Separation of the polymer beads after graft-polymerization can occuraccording to methods known in the art, e.g. by spray-drying,centrifuging, etc. Other separation methods can be used also, e.g.evaporation of the solvent medium. But in this case there exists somedanger of conglomeration of the polymer particles, which may necessitatean additional fine grinding of the mass.

The separated polymer beads can be added e.g. to the coating compositionfor a photographic layer. Of course, the usual coating additives can beadded to this coating composition.

Sometimes, it may not be necessary, however, to separate the polymerbeads from the liquid phase. If desired, they can indeed be added as adispersion in the liquid phase to a coating composition for aphotographic layer. Such coating composition can be coated as such onthe rear side of a photographic element or, as the case may be, on topof a photosensitive silver halide emulsion layer.

The photographic layer containing polymer beads in accordance with thepresent invention has a thickness, which--depending on the purpose ofsaid layer--varies between about 0.5 and about 3 μm. The thickness ofsaid layer is lower than the average size of the polymer beads so thatin fact a large number of these polymer beads protrude from the layer.For instance in a layer having a thickness of 0.5 to 1.0 μm, beadssizing 1.0 to 2.0 μm can be used.

Thanks to the presence of these protruding polymer beads in aphotographic layer of a photographic element, contact between the latterlayer and other surfaces only exists at the smooth tops of theprotruding beads and these other surfaces. In this way the dry frictionsurface is greatly reduced so that abrasion or scratching andconsequently dust formation owing to mechanical resistance are highlydiminished.

The photographic layer comprising the polymer beads as described abovecan serve several purposes.

For instance, the photographic layer can be an antifriction surfacelayer, which reduces dry friction of the photographic element againstcontacting objects and protects the photographic element againstexternal influences. It can be coated at the rear side of the filmsupport or on the uppermost photosensitive silver halide emulsion layer.The layer can also be a carbon black antihalation layer coated on therear side of the photographic element, which carbon black layer iseliminated from the support during the processing sequence or said layercan be a coloured antihalation layer that is discoloured in a processingbath and remains on the rear side of the photographic film support. Thelayer can also be an antistatic surface layer that contains the polymerbeads. The antistatic surface layer can be coated on an antihalationlayer that is eliminated or discoloured during processing. Furthermore,the layer containing the polymer beads can also be a protective surfacelayer coated over the photosensitive emulsion layer(s) or at the rearside of the film support, said protective surface layer comprising ahydrophilic colloid e.g. gelatin and said polymer beads dispersedtherein as matting agents. It is also possible to use the polymer beadsin a top layer of a drafting film, said top layer facilitating writingor drafting thereon. A further interesting application of the polymerbeads is in a subbing layer coated on a film support for preventing thesurface of the latter during winding up of the subbed film support fromsticking to the rear surface of the same or for improving the transportof the subbed film support in a coating machine e.g. during the coatingthereon of other layers e.g. silver halide emulsion layers.

When the layer comprising the polymer beads is an antifriction surfacelayer, it protects the photographic element and more particularly thesupport or the uppermost photosensitive silver halide emulsion layeragainst mechanical damage resulting from dry friction against contactingobjects and as a result thereof it reduces abrasion and formation ofdust. The polymer beads are in dispersed form in a colloid and partiallyprotrude from the surface of the colloid layer.

When the layer comprising the polymer beads has the function of anantihalation layer, it has been applied to the side of the supportopposite to that carrying the photosensitive silver halide emulsionlayer(s). The layer then contains antihalation dyes or pigments inaddition to the polymer beads. Such antihalation layer in dry conditionbefore the processing sequence thus advantageously prevents thephotosensitive silver halide layer(s) from sticking to the film supportwhenever the photographic element is in wound up or stacked conditionand at the same time efficiently prevents light that during theimage-wise exposure penetrates through the photosensitive silver halideemulsion layer(s) from being reflected by the support and influencingthe emulsion layer(s) for a second time.

According to a preferred embodiment of the present invention anantihalation surface layer of a photographic element comprises i.a. awater-insoluble, alkali-soluble, polymeric binder, an antihalation dyeor pigment that absorbs the light penetrating through the emulsionlayer(s), and the polymer beads as described herein. This antihalationsurface layer can be eliminated entirely from the support of thephotographic element during one of the first steps of the processingsequence. For instance, the antihalation surface layer containing analkali-soluble polymeric binder is pretreated in an alkaline prebath andis removed from the rear side of the support in the following waterbathwith the aid of rubbing means e.g. rotating brushes during rinsing e.g.spray rinsing, immediately subsequent to the treatment with the alkalineprebath.

More details about water-insoluble, alkali-soluble polymeric bindersthat can be used advantageously in an antihalation surface layer as setforth above and other features of antihalation coatings can be found inthe U.S. Pat. No. 3,853,563. Preferred water-insoluble, alkali-solublebinders are co(styrene/acrylic acid), co(vinyl acetate/maleic acid),co(ethyl acrylate/methyl methacrylate/methacrylic acid), etc.

The above polymers in their acid form are insoluble in water butdissolve readily in their ionic form so that the antihalation surfacelayer containing them disintegrates and can be eliminated easily orloosens from the photographic film support in the waterbath.Consequently, a clear photographic film remains.

In order to obtain the desired absorption spectrum e.g. absorption ofall light of the visible spectrum in the antihalation surface layer, oneor more known light-absorbing pigments or dyes can be used e.g. carbonblack, triphenyl methane dyes, etc.

The coating composition for the antihalation surface layer may furthercomprise one or more surface-active agents e.g. of the type described inU.S. Pat. No. 2,600,831, 3,026,202, and 3,663,229, in Belgian Pat. No.742,680 and in European published Patent Application No. 00 15 592,sizing agents, waxes, etc.

The coating composition for the antihalation surface layer can beapplied to the film support according to known methods. Examples of filmsupports are films of cellulose triacetate, polyalkylene terephthalatee.g. polyethylene terephthalate, and polycarbonates.

In certain cases it may be advisable to provide the support with aprimer coating or a subbing layer before the application of theantihalation surface layer or to pretreat the support superficiallyaccording to known techniques such as an electrical treatment with ahigh voltage corona, etc. An interesting primer coating or subbing layerfor application between a polyethylene terephthalate support and theantihalation surface layer has been described e.g. in the U.S. Pat. No.4,132,552.

In order to facilitate the elimination of the antihalation surface layerin an alkaline processing bath, there may be applied between the filmsupport and the antihalation surface layer an intermediate layer, whichhas been formed from a mixture of 1 to 3 parts by weight of a celluloseester, e.g. cellulose diacetate and 3 to 1 part by weight of at leastone alkali-soluble polymer as referred to above. More details about suchintermediate layers can be found in the Belgian Patent Specification No.773,588.

The thickness of the antihalation surface layer is not critical, thoughgenerally a thickness of 0.5 to 3 μm as mentioned above is used. Thethickness of the layer and the amount of pigment or dye are preferablyregulated so that the resulting layer has an optical density of about0.5 to 1.5.

According to a preferred embodiment of the present invention the polymerbeads prepared according to the method of the present invention are usedin an antihalation surface layer at the rear side of a photographicelement comprising at least one photosensitive silver halide emulsionlayer and a cellulose triacetate support. The latter antihalationsurface layer comprises polymer beads prepared and composed as describedabove and sizing from 1.8 to 3 μm with an average diameter of about 2.2μm as well as carbon black as antihalation pigment and awater-insoluble, alkali-soluble binder. This antihalation surface layeris coated at a thickness of about 1 μm so that the polymer spheresprotrude from its surface and consequently act as spacing agents, whichoffer the advantage of preventing the carbon black from entering intocontact with other surfaces. Soiling of rollers or other contactingsurfaces with carbon black is thus avoided adequately. This antihalationsurface layer can be applied advantageously in cinematographic colourmaterials.

For graphic arts materials it is also possible to use the polymer beadsdescribed above in an antihalation surface layer comprising ahydrophilic colloid as binder and an antihalation dye that can bediscoloured in a processing bath. In that case the discolouredantihalation surface layer remains on the photographic film supportafter processing of the photographic element. Gelatin, casein, polyvinylalcohol, poly-N-vinyl pyrrolidone, sodium alginate, sodiumcarboxymethylcellulose etc. can be used as hydrophilic colloid, gelatinbeing preferred, however.

Since in this case the antihalation surface layer, after having lost itsantihalation function by discolouration of the antihalation dye duringprocessing, remains permanently on the support, it may give rise toadditional advantages. As a matter of fact the remaining surface layermay reduce the sticking tendency of the photographic element againstother surfaces due to the diminished contact surface. This limitedcontact area may also result in less tendency towards the formation ofNewton's rings and towards the generation of static electricity.

The polymer beads used in a photographic element in accordance with theinvention can also be incorporated in an antistatic surface layer ofsaid photographic element, the protruding beads reducing the contactsurface between said photographic element and the contacting objects soas to prevent the generation of static electricity, which may causesparks and thus exposure of the photographic element. It is alsopossible to apply an antistatic surface layer comprising the polymerbeads on an antihalation layer at the rear side of the film support of aphotographic element e.g. a photographic microfilm element, theantihalation layer dissolving completely and discolouring duringdevelopment of the photographic element so that nothing remains at therear side of the film support.

Another interesting application of the polymer beads in accordance withthe invention is the use thereof as matting agents in a protectivesurface layer coated on the uppermost photosensitive silver halideemulsion layer of a photographic element and/or at the rear side of aphotographic element. The dispersions of polymer beads can beincorporated by stirring into an aqueous colloid composition e.g. agelatin composition, which can be provided with the usual coatingadditives, to form the coating composition for said protective surfacelayer. The polymer beads protrude from the protective surface layer andconsequently give it a mat and rough appearance. They do not have adeleterious or adverse influence on the photosensitive silver halideemulsion layer(s). The antistatic properties of the photographicelements containing the polymer beads as matting agents in (a)protective surface layer(s) are improved. The protective surface layercan be applied to any type of black-and-white or colour photographicsilver halide emulsion layer, to a filter layer, to an antihalationlayer or to an anti-curling layer.

Another successful use of the polymer beads according to the inventionis as matting agents in a surface layer of a drafting film containing aphotosensitive layer e.g. a wash-off film.

Although the invention has been described with particular reference tolayer(s) of a photosensitive silver halide element, the polymer beads ofthe invention can also be used in the layer(s) of other materials e.g.in the surface layer(s) of a film support, of photopolymerizationmaterials, of diazotype materials, of thermographic materials, etc.

The polymer beads according to the invention can also be incorporatedinto polymer films, preferably into a polyethylene terephthalate film,for the purpose of improving the winding up and the storagecharacteristics, in other words to avoid sticking of the rear surface ofan untreated film against the adjacent top surface of said film whenwound up on reels. For this purpose the polymer beads can be separatedfirst from the bead dispersion as described above, then ground ifdesired, and finally added in dry or in wet state according to methodsknown in the manufacture of polyester film, to the raw materials neededtherefor. In this way the polyethylene terephthalate film made accordingto the usual process comprising stretching and heat-setting acquiresfavourable winding up and friction characteristics, whereas the opticalcharacteristics can remain almost unaltered. The thus obtained film canbe used as the support of a photographic element comprisinglight-sensitive layers or it can be employed for other purposes. Theaddition of polymer beads can be to the total mass of polyethyleneterephthalate granules or to a part thereof, which part can then beextruded on or co-extruded with the other part of polyethyleneterephthalate so as to form a stratum at the surface of the resultingfilm. The part of polyethylene terephthalate granules containing thepolymer beads according to the invention can also be applied bylamination onto the extruded polyethylene terephthalate film so as toform a bead-containing stratum thereon. The amount of polymer beadsadded to the polyethylene terephthalate should normally not exceed 1% byweight of the total weight of polyethylene terephthalate and polymerbeads. Preferably, however, 0.01`%-0.05% by weight of polymer beads isused in respect of the total weight. The preferred size of the polymerbeads incorporated into a film is between 1 and 5 μm.

The preparation of polymer beads for use in accordance with theinvention is illustrated in the following preparation examples, theaverage size of the polymer beads stated therein being determined with 2different instruments, both being marketed by Coulter Electronics Ltd.,Coldharbour Lane, Harpenden, Hertfordshire, AL 54 UN, United Kingdom.

The first instrument is the COULTER COUNTER (registered trade mark)Model TA II particle size analyser. The Coulter principle is based on anelectric path of small dimensions, which is modulated by momentarypassage of each particle one-by-one. Particles suspended in anelectrolyte are forced through a small aperture, across which anelectric current path has been established. Each particle displaceselectrolyte in the aperture producing a pulse equal to its displacedvolume. Thus, three dimensions, or particle volume response is the basisfor all measuring. The average size of the polymer beads versus theirrelative volume (weight) or number is given by the instrument. Therecorder plots histograms on number and weight basis.

The second instrument is the COULTER (registered trade mark) NANO-SIZER.The measuring principles used in this instrument are those of BrownianMotion and autocorrelation spectroscopy of scattered laser light. Thefrequency of this Brownian Motion is inversely related to particle size.The instrument also computes a polydispersity index, which is a measureof the width of the size distribution. For instance an index of 0 or 1would describe an essentially monosized distribution, whereas 8 or 9would describe a wide range distribution.

PREPARATION 1: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND CO(STYRENE/MALEIC ACID MONOSODIUMSALT)

At room temperature 1566 g of a 10% aqueous solution ofco(styrene/maleic acid anhydride) adjusted to pH 7.0 by means of sodiumhydroxide, 4617 ml of distilled water, and 48.6 g (12.5×10⁻³ mol perliter of reaction medium) of potassium persulphate were placedsuccessively in a 20.0 liter reaction vessel equipped with a stirrer, athermometer, and a nitrogen inlet above the liquid level. During theentire reaction the atmosphere in the reaction flask was rinsedcontinuously with nitrogen to keep it free from air.

The mixture was stirred constantly at 140 rpm. After 10 minutes ofstirring, the persulphate had dissolved and 5400 ml of ethanol and 3192ml (3.0 kg) of methyl methacrylate were added at once.

Stirring was then continued for 90 minutes at room temperature. Thereaction mixture remained turbid all the time.

Next, the reaction mixture was heated gradually with a waterbath at 65°C. As soon as the temperature in the reaction flask reached 30° C., thereaction mixture became transparent.

At a temperature of 55° to 60° C. the first turbidity was usually seen.After a total heating time of 30 minutes the temperature in the reactionvessel reached 65° C.

As a consequence of the exothermic polymerization reaction thetemperature rose gradually to 80° C. At this very moment a weak refluxtook place.

The increase in temperature from 60° to 80° C. took almost 45 minutes.During this period the clear solution changed into a milky whitedispersion.

The temperature remained for almost 5 minutes a 80° C. and then startedfalling gradually to 65° C. in about 30 minutes.

Subsequently, the dispersion was stirred for 16 hours on the waterbathat 65° C.

After the polymerization the dispersion was cooled to 30° C. withstirring.

Finally, the dispersion was filtered through a nylon cloth with meshessizing 75×75 μm.

Yield: 13.19 kg of dispersion of polymethyl methacrylate beadsstabilized with a graft copolymer of methyl methacrylate andco(styrene/maleic acid monosodium alt) comprising 23.9 g of beads per100 g of dispersion (yield of 98.4%) at pH 5.2. The average size of thepolymer beads measured with the aid of the COULTER NANO-SIZER was 2.190μm, the polydispersity index being 0. The COULTER COUNTER Model TA IIgave an average size of the beads of 2.02 μm when measured in numberpercent and of 2.09 μm when measured in weight percent.

PREPARATION 2: POLYSTYRENE BEADS STABILIZED WITH A GRAFT COPOLYMER OFSTYRENE AND CO(STYRENE/MALEIC ACID MONOSODIUM SALT)

At room temperature 82.72 g of an aqueous solution containing per 100 g,10.0 g of co(styrene/maleic acid monosodium salt) and adjusted to pH 7.0with sodium hydroxide, next 226.9 ml of demineralized water, and finally1.65 g (12.5×10⁻³ mol per liter of reaction medium) of potassiumpersulphate were brought at room temperature in a 2.0 liter reactionflask equipped with a stirrer, a thermometer, and a nitrogen inlet abovethe liquid level. During the entire reaction the atmosphere in the flaskwas rinsed continuously with nitrogen to keep it free from air.

The mixture was stirred constantly at 140 rpm. After 10 minutes ofstirring, the persulphate had dissolved and 165.5 g of styrene and 670ml of ethanol were introduced at once into the flask.

Stirring was continued at room temperature for 1 h. Next, the reactionflask was heated by means of a waterbath at 70° C.

At 50° C. the solution became clear and at 55° C. the first turbiditywas seen.

When the temperature in the reaction flask reached 65° C., thetemperature of the waterbath was lowered from 70° C. to 65° C.

A milky white dispersion formed. Stirring at 65° C. was continued for 18h.

The dispersion was heated for 2 hours more at 80° C. and finally, thepolymer dispersion was filtered through a nylon cloth with meshes sizing70×70 μm. Yield: 918 g of dispersion comprising 18.6 g of polystyrenebeads stabilized with a graft copolymer of styrene and co(styrene/maleicacid monosodium salt) per 100 g of dispersion (yield of 97.4%) of pH6.0.

The average size of the polymer beads measured with the COULTERNANO-SIZER was 1.99 μm, the polydispersity index being 1.

PREPARATION 3: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND POLYETHYLENE OXIDE

At room temperature 52.0 g of polyethylene oxide having a molecularweight of 20,000 was dissolved in a mixture of 2009 g of demineralizedwater and 1800 ml of ethanol in a 5.0 liter reaction vessel equippedwith a stirrer and a reflux condenser.

The solution was stirred constantly at 140 rpm and rinsed with nitrogenduring the entire reaction.

An amount of 16.2 g of potassium persulphate was added to the solutionand dissolved also at room temperature.

After about 10 min 1000 g of methyl methacrylate was added at once tothe clear solution. The slightly opaquing solution was stirred foranother 90 min at room temperature.

Next, the opaque solution was heated on a water-bath at 70° C. At 30° C.the solution became completely transparent, but at about 40° C. theclear solution again became opaque as a result of the initiated beadformation and at 60° C. it turned into a milky white dispersion.

Since the polymerization reaction was slightly exothermic, thetemperature of the reaction medium rose to 73° C. The exothermic phaselasted approximately 40 min. Afterwards the temperature of the reactionmedium fell to about 67° C. The dispersion was stirred then for another18 h on the water-bath of 70° C. at 140 rpm. The temperature of thedispersion remained at approximately 67° C.

Finally, the dispersion was filtered through a nylon gauze with a meshwidth of 75×75 μm.

Yield: 4520 g of dispersion containing 23.5 g of polymer beads per 100 gof dispersion at pH 5.3.

The average size of the polymer beads measured with the COULTERNANO-SIZER was 1.5 μm, the polydispersity index being 0. The COULTERCOUNTER Model TA II gave an average size of the beads, when measured innumber percent, of 1.54 μm and, when measured in weight percent of 1.98μm.

PREPARATION 4: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND CO(ACRYLIC ACID/STYRENE SODIUMSALT)

At room temperature 52.2 g of co(acrylic acid/styrene) comprising 36.5%by weight of acrylic acid and 63.5% by weight of styrene and having anintrinsic viscosity [η]=0.30 dl/g in methanol at 25° C. was dissolved in250 ml of demineralized water and 750 ml of ethanol and then adjusted topH 7.5 with 5N sodium hydroxide in a reaction vessel equipped with astirrer, a thermometer, and a nitrogen inlet above the liquid level.After the copolymer a volume of 1829 ml of demineralized water and anamount of 16.2 g of potassium persulphate were placed successively inthe reaction vessel. During the entire reaction the atmosphere in thereaction vessel was rinsed continuously with nitrogen to keep it freefrom air.

The mixture was stirred constantly at 140 rpm. After 10 min of stirring,the persulphate had dissolved and 1050 ml of ethanol and 1000 g ofmethyl methacrylate were added at once.

The procedure described in Preparation 1 was then followed. Yield: 4435g of dispersion comprising 24.0 g of polymer beads per 100 g ofdispersion at pH 5.2.

The average grain size of the polymer beads measured with the COULTERNANO-SIZER was 1.92 μm, the polydispersity index being 0. The COULTERCOUNTER Model TA II gave an average size of the beads of 1.13 μm whenmeasured in number percent and of 2.20 μm when measured in weightpercent.

PREPARATION 5: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND CO(VINYL ALCOHOL/VINYL ACETATE)

At room temperature 52.2 g of co(vinyl alcohol/vinyl acetate) comprising60% by weight of vinyl alcohol and 40% by weight of vinyl acetate andhaving an intrinsic viscosity [η]=0.20 dl/g in water at 25° C. wasdissolved in 2009 ml of demineralized water and 1800 ml of ethanol in a5.0 liter reaction vessel equipped with a stirrer and a refluxcondenser.

The solution was stirred constantly at 140 rpm during the entirereaction.

An amount of 16.2 g of potassium persulphate was added to the solutionand dissolved also at room temperature.

After about 10 min 1000 g of methyl methacrylate was added at once tothe clear solution. The procedure described in preparation 3 was thenrepeated.

After filtration 4435 g of dispersion containing 24.0 g of polymer beadsper 100 g of dispersion was obtained.

The average size of the polymer beads measured with the COULTERNANO-SIZER was 2.32 μm, the polydispersity index being 1. The COULTERCOUNTER Model TA II gave an average size of the beads of 1.40 μm whenmeasured in number percent and of 2.80 μm when measured in weightpercent.

PREPARATION 6: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND POLY-N-VINYL PYRROLIDONE

At room temperature 52.2 g of poly-N-vinyl pyrrolidone having amolecular weight of 40,000 was dissolved in 2009 ml of demineralizedwater and 1800 ml of ethanol in a 5 liter reaction vessel equipped witha stirrer and a reflux condenser.

The solution was stirred constantly at 140 rpm during the entirereaction.

An amount of 16.2 g of potassium persulphate was added to the solutionand dissolved also at room temperature.

After about 10 min 1000 g of monomethyl methacrylate was added at onceto the clear solution. The procedure described in preparation 3 was thenrepeated.

After filtration 4470 g of dispersion at pH 5.0 containing 24.0 g ofpolymer beads per 100 g of dispersion was obtained. The average size ofthe polymer beads measured with the COULTER COUNTER NANO-SIZER was 0.564μm, the polydispersity index being 3.

PREPARATION 7-12

In this series of preparation examples 7 to 12 the preparation procedureand the reaction components described in preparation 1 were used. Thisseries of examples was planned to illustrate the influence of thequantitative proportion between the solvents of the solvent mixture onthe size of the polymer beads obtained. The results are listed in thefollowing table 1. The results of preparation example 1 have been takenover also therein to facilitate comparison. The average size of thebeads was measured with the COULTER NANO-SIZER as well as with theCOULTER COUNTER Model TA II (average size measured by number percent andaverage size measured by weight percent).

                                      TABLE 1                                     __________________________________________________________________________    ml of                                                                         solvents   quantitative                                                                          average bead size (dm) in μm                            added to re-                                                                             proportion in                                                                         measured by                                                prep.                                                                            action medium                                                                         % by volume of  COULTER COUNTER in                                 ex.                                                                              water                                                                             ethanol                                                                           water/ethanol                                                                         NANO-SIZER                                                                            number %                                                                             weight %                                    __________________________________________________________________________    7  3161                                                                              6856                                                                              40/60   amorphous precipitate with beads                                              up to 0.1 μm                                            8  3504                                                                              6513                                                                              43/57   2.675 (0)                                                                             1.38   3.90                                        9  3618                                                                              6399                                                                              44/56   2.375 (0)                                                                             1.62   3.92                                        10 3846                                                                              6171                                                                              46/54   2.380 (0)                                                                             2.34   2.79                                        1  4617                                                                              5400                                                                              52.75/47.25                                                                           2.190 (0)                                                                             2.02   2.09                                        11 5446                                                                              4571                                                                              60/40   1.400 (0)                                                                             0.85   0.98                                        12 6589                                                                              3428                                                                              70/30    1.100 (3-4)                                                                          0.81   2.36                                        __________________________________________________________________________

In the above table 1 after the value of the average bead size(NANO-SIZER) the polydispersity index is given between parentheses.

From the results of table 1 it can be learned that an increase in theamount of water as solvent in the solvent mixture from 43 to 60% byvolume causes the average bead size to decrease considerably(preparation examples 7, 8, 9, 10, 1, and 11).

According to the preparation examples 11 and 12 it was found that uponfurther increasing the amount of water from 60 to 70% by volume theaverage bead size did not change considerably, but that essentially onlythe homodispersity was adversely affected.

The diagrams given in the accompanying figures numbered 1, 2, 3, and 4show the bead size distribution curves of the polymer beads obtainedaccording to the preparation examples 1, 8, 11, and 12 respectively.These distribution curves were obtained with the COULTER COUNTER ModelTA II. The abscissa of each diagram represents the size of the polymerbeads in micrometer and the ordinate represents the relative volume(weight) or number of the polymer beads.

                                      TABLE 2                                     __________________________________________________________________________    Amount of C   ml of H.sub.2 O                                                     in g      (demin.)                                                            of C                                                                             in g of 10%                                                                          added to                                                            per                                                                              aqueous solu-                                                                        the reac-                                                                           g of amorphous preci-                                                                          filtrability through                     Prep.                                                                             100 g                                                                            tion of C                                                                            tion  pitate per 100 g of                                                                      bead size                                                                           nylon gauze having a                     ex. of D                                                                             (pH 7) medium                                                                              C + D      in μm                                                                            mesh width of 75 × 75              __________________________________________________________________________                                         μm                                    13  0    0    6027  amorphous precipitate                                                                    --    --                                                           comprising no beads                                                           at all                                                    14  1.3                                                                                391.5                                                                              5675  not determined                                                                           10-50 not filtrable                                                           (irregular                                                                    form)                                          15  2.6                                                                               783   5322  3.2        2.07  fairly filtrable; but                                                         upon prolonged standing                                                       forming a hard cake                       1  5.2                                                                              1566   4617  <0.5       2.19  fairly filtrable; a soft                                                      precipitate after pro-                                                        longed standing, which                                                        can be redispersed by                                                         simple stirring                          16  7.8                                                                              2340   3921  0          1.61  hardly filtrable as a                                                         result of the increased                                                       viscosity                                17  10.0                                                                             3000   3327  not determined; for-                                                                     --    --                                                           mation of large                                                               amounts of amorphous                                                          precipitate                                               __________________________________________________________________________     C refers to the graftpolymerizable copolymer co(styrene/maleic acid sodiu     salt)                                                                         D refers to the monomer methyl methacrylate.                             

The solid line curves pertain to the number percent of the polymerbeads, the dash line curves to the weight percent. For instance, in FIG.1 point A on the solid line curve indicates that in a number of 100polymer beads 90 are larger than (oversize) 0.8 μm and point B on thedash line curve indicates that per 100 g of polymer beads 90 g of beadsare larger than (oversize) 1.8 μm. The same points A and B, when read interms of undersize, correspondingly indicate that 10 number percent ofthe beads are smaller than 0.8 and 1.8 μm respectively. The averageparticle size in % by number or % by weight is determined at the pointcorresponding with 50% undersize/oversize.

PREPARATIONS 13-17

Preparations 13-17 illustrate the influence of the amount of graftpolymerizable (co)polymer on the formation of beads, more particularlyon the size of the beads, their stability, and the amount of undesiredamorphous precipitate formed.

The preparations 13-17 were carried out in an analogous way to thatdescribed in preparation example 1, with the only difference, however,that the amount of 10% aqueous solution of co(styrene/maleic acidanhydride) (neutralized with sodium hydroxide to pH 7.0) was modifiedand that the amount of water added afterwards was adapted as indicatedin the following table 2. The bead size was measured by means of theCOULTER NANO-SIZER.

PREPARATIONS 18-21

In the foregoing preparation examples an amount of 12.5×10⁻³ mol ofpotassium persulphate initiator was used always per liter of reactionmedium, sodium or ammonium persulphate giving the same results.

It seemed advisable to check the relation between the amount ofinitiator and the final size of the polymer beads obtained. A series ofpreparation examples 18 to 21 was made therefore. In these preparationexamples 18 to 21 the same preparation procedure and the same reactioncomponents as those employed in preparation example 1 were used, withthe only difference, however, that the amount of potassium persulphateinitiator was modified as indicated in the following table 3.

The results listed in table 3 show that in comparison with the amount ofinitiator used in example 1 a reduced amount of initiator leads togrowing beads and that an increased amount of initiator results in theformation of smaller beads. In order to facilitate comparison ltheresults of preparation example 1 have been taken over also in table 3.

                  TABLE 3                                                         ______________________________________                                               × 10.sup.-3                                                      Prepara-                                                                             mol/liter               bead                                           tion   of reaction                                                                             evaluation of size  pH of                                    example                                                                              medium    dispersed beads                                                                             in μm                                                                            dispersion                               ______________________________________                                        19      1.0      amorphous precipi-                                                            tate without dis-                                                             persed beads                                                 18      5.0      amorphous precipi-  7.1                                                       tate without dis-                                                             persed beads                                                  1     12.5      regular homodis-                                                                            2.15  5.1                                                       perse beads                                                  20     20.0      regular homodis-                                                                            2.16  4.9                                                       perse beads                                                  21     40.0      regular homodis-                                                                            1.79  3.3                                                       perse beads                                                  ______________________________________                                    

It follows that between 12.5×10⁻³ and 40×10⁻³ mol of potassiumpersulphate per liter of reaction medium regular homodisperse beadssizing from 2.36 to 2.15 μm can be made with otherwise constant reactioncomponents and parameters. Beads of these sizes are very apt for use inan antifriction antihalation surface layer containing carbon black asantihalation pigment.

It was also found that a very high amount of persulphate initiator e.g.40×10⁻³ mol, besides giving a reduced bead size, also caused thepH-value of the dispersion to fall, so that upon dilution with water,the beads started conglomerating. Only by addition of sodium hydroxideup to a pH of about 6.0 could the beads be re-dispersed.

PREPARATION 22: POLYMETHYL METHACRYLATE BEADS STABILIZED WITH A GRAFTCOPOLYMER OF METHYL METHACRYLATE AND CO(STYRENE/-2-ACRYLOYLOXYETHYLMONOMALEINATE/MALEIC ACID SODIUM SALT)

First the copolymer having reactive side groups, viz.co(styrene/2-acryloyloxyethyl monomaleinate/maleic acid sodium salt) wasprepared as follows, the proportion of the monomers being 50/5/45 mol %.

In a 1 l reaction vessel equipped with a stirrer, a reflux condenser,and a nitrogen inlet 101 g of co(styrene/maleic anhydride) and 0.1 ml of0.6% solution of picric acid in acetone were dissolved with stirring atroom temperature in 420 ml of acetone. The co(styrene/maleic anhydride)had an equimolar composition and an intrinsic viscosity of 0.32 dl g⁻¹in acetone at 25° C.

An amount of 5.8 g of 2-hydroxyethyl acrylate was added to the solution,which was then stirred for 6 h at 50° C.

As soon as the reaction came to an end, the solution was diluted withacetone to make 1200 ml and then poured out gradually in 12 l of waterwith continuous stirring. The fibrous copolymer was filtered off anddried in a ventilated drying oven at 40° C. until the weight remainedconstant.

Yield: 105 g of co(styrene/2-acryloyloxyethyl monomaleinate/maleic acidsodium salt).

Next, 16.7 g of the resulting copolymer were dissolved in 500 ml ofwater with gradual addition of 5N sodium hydroxide. The pH-value of thesolution was adjusted to 7.5. The solution was filtered and diluted withwater to make 588 g solution.

The resulting solution was placed in a 2 l reaction flask equipped witha stirrer, a nitrogen inlet, and a reflux condenser. An amount of 5.4 gof potassium persulphate was dissolved therein and 698 ml of ethanol and333.3 g of methyl methacrylate were added.

The homogeneous solution was rinsed with nitrogen for 2.5 h at roomtemperature with stirring at 140 rpm and then heated on a water-bath of65° C.

As soon as the reaction medium attained 63° C. the polymerization becameslightly exothermic, the solution then transforming into a milky whitedispersion. The temperature continued rising to 73° C. Afterwards thetemperature started falling gradually to 63° C. The dispersion wasstirred continuously for 16 h at this temperature. Finally, thedispersion was cooled down to room temperature and filtered through anylon cloth having a mesh width of 120×120 μm.

Yield: 1356 g of dispersion comprising 24.0 g of polymer beads per 100 gof dispersion. The average size of the polymer beads measured with theCOULTER COUNTER NANO-SIZER was 2.36 μm, the polydispersity index being0.

The application of the polymer beads for use in accordance with theinvention is illustrated in the following examples.

EXAMPLE 1

A cellulose triacetate film support was coated on its front side with agelatin subbing layer and on its rear side with anticurling layeressentially consisting of cellulose diacetate. The anticurling layer wascovered according to the reverse roller coating system at a ratio of 20sq.m/l with the following antihalation coating solution:

25% aqueous dispersion of co(ethyl acrylate/methylmethacrylate/methacrylic acid) (50/33.5/16.5): 54 ml

water: 334 ml

methanol: 550 ml

1N ammonium hydroxide: 20 ml

20% aqueous dispersion of carbon black: 25 ml

20% dispersion in water/methanol of wax containing per liter: 14 ml

11.57 g of a mixture of ##STR1## in which alkyl is C₁₅ -C₂₀ alkyl and nis 15 to 20 23.57 g of ##STR2## wherein x+y=4 96.4 g of paraffin wax

69.4 g of polyethylene

20% dispersion in equal volumes of ethanol and water of polymer beadsconsisting of polymethyl methacrylate and the graft copolymerco(styrene/maleic acid monosodium salt) and having an average size ofabout 2.2 μm according to preparation 1: 3 ml.

After drying with hot air the support carrying the antihalation layerwas tested as follows and measurements were carried out as follows.

An adhesive tape was pressed tightly onto the carbon black antihalationlayer and then torn off at once. The carbon black layer was not damagedby the adhesive tape and it remained adhering strongly and completely tothe support.

The film support with its antihalation layer turned upside was thenplaced on a flat plate and attempts were made to scratch theantihalation layer with the fingernail. The antihalation layer resistedsuccessfully, however.

The dynamic friction coefficient of the antihalation layer as comparedwith that of stainless steel was found to be 0.18 to 0.20 as measuredwith the device described in Jnl. of Scientific Instruments, Volume 28,July 1951, page 220.

The lateral resistance measured at a relative humidity of 60% was 3×10⁷ohm/cm2.

The optical density of the antihalation layer was found to be 0.95.

The antihalation layer complied with all requirements concerning theprocessing of cinematographic colour materials as described in "Abridgedspecifications for Process ECP-2, Kodak Publication No. H-37". Theantihalation layer ran through the pre-bath without soiling it and waseliminated entirely during rinsing.

The splicing of processed film, which had been coated previously withthe carbon black antihalation layer, was not adversely affected.

The behaviour of the carbon black antihalation surface layer at the rearside of the film support during the coatin of emulsion layers on thefront side of the film support, in other words during the running of theantihalation layer over the various transport rollers of the emulsioncoating machine, was simulated and checked in a dust-releasing test byplacing a piece of the film support sizing 24×50 cm, the antihalationsurface layer turned upside, on a flat plate and rubbing theantihalation surface layer under slight finger pressure with a piece offilter paper sizing 2×2 cm 100 times to and fro in such a way thatfinally the whole surface of the antihalation layer had been rubbed.

The density of the filter paper was then evaluated as follows. If thefilter paper had been soiled during the rubbing of the antihalationlayer only to a very little extent, it was accepted that the filmsupport could be conveyed through the emulsion-coating device withoutgetting the transport rollers soiled by the antihalation layer. If,however, the filter paper would have been blackened considerably duringthe rubbing test, the transport rollers of the emulsion coating devicewould also have been soiled by the antihalation layer. In the case of anantihalation layer composed as described above in this example 1, thefilter paper was soiled but very weakly.

A magnetic sound track paste containing iron(III) oxide as described inthe U.K. Patent Specification No. 1,507,983 could be applied easily tothe antihalation layer. The magnetic paste adhered very well before aswell as after the processing.

A film material comprising the film support and the antihalation layerdescribed above was stored for 24 h at 25° C. and a relative humidity of60% and then rolled up air-tight and damp-proof the emulsion sideagainst the antihalation side.

After a storage in this condition of 3 days at 57° C., no transfer ofblack antihalation layer to the emulsion layer or vice-versa could beobserved after unrolling.

EXAMPLES 2-4

For comparison with the results of example 1 with the polymer beadsaccording to the invention, 3 strips of film support as described inexample 1 were covered with the same antihalation coating solution asdescribed therein, with the only difference that the dispersion ofpolymer beads was replaced each time by an equivalent amount of one ofthe following grains or particles:

(a) silica particles having an average size of 3.3 μm,

(b) finely divided grains of a hydrophobic reaction product of starchand urea formaldehyde sizing from 5 to 6 μm as described in the U.K.Patent Specification No. 985,115,

(c) urea formaldehyde grains containing silica and having an averagesize of 2 μm, as described in European Patent Application No. 0,003,627.

During a dust-releasing test as described in example 1 with the 3 stripscontaining the grains (a), (b), or (c) considerably higher amounts ofblack dust came loose than with the film support carrying the polymerbeads as described in example 1.

EXAMPLE 5

A polyethylene terephthalate film support having a thickness of 120 umwas coated successively with a subbing layer composition consisting of acopolyester of ethylene glycol with iso- and terephthalic acid and5-sulpho-isophthalic acid sodium salt and with a coating composition fora carbon black-containing antihalation surface layer at a ratio of 1 lper 35 sq.m according to the air-knife coating system, the coatingcomposition for the antihalation surface layer containing:

demineralized water: 674.4 ml

methanol: 100 ml

5% solution of tetraethylammonium perfluorooctyl sulphite in a mixtureof demineralized water and methanol (50:50): 3 ml

resorcinol: 6.5 g

1N ammonium hydroxide: 40 ml

50% aqueous dispersion of paraffin wax: 5.85 ml

60% aqueous dispersion of poly(tetrafluoroethylene): 3.25 ml

20% aqueous dispersion of carbon black: 40.5 ml

25% aqueous dispersion of co(ethyl acrylate/methylmethacrylate/methacrylic acid) (25/50/25): 128 ml

20% dispersion in equal volumes of ethanol and water of the polymethylmethacrylate beads described in example 1: 5 ml

The dust-releasing test (described in example 1) with this antihalationsurface layer revealed that almost no dust came off. The other testsdescribed in example 1 were repeated with the present antihalationsurface layer and also revealed very satisfactory results.

EXAMPLE 6

A biaxially oriented polyethylene terephthalate film support having athickness of 63 μm was coated on its front side with a subbing layer andon its rear side at a ratio of 1 l per 60 sq.m with the followingcoating composition for an antihalation layer:

co(styrene/acrylic acid)(70/30): 40 g

triphenylmethane dye corresponding to the formula: 20 g ##STR3##triphenylmethane dye corresponding to the formula: 13 g ##STR4##methanol: 200 ml ethanol: 200 ml

acetone: 600 ml

After having been dried the antihalation layer was covered at a ratio of1 l per 28 sq.m with the following coating composition for an antistaticsurface layer:

co(styrene-sodium maleate)(50/50): 4.8 g

10% aqueous dispersion of polyethylene: 12 ml

10% aqueous solution of a compound corresponding to the followingformula: 5.6 ml ##STR5## methanol: 190 ml a dispersion in water/ethanolof polystyrene beads stabilized with a graft copolymer of styrene andco(styrene/maleic acid monosodium salt) prepared as described inpreparation 2 hereinbefore: 1.2 ml

water to make: 1 l

The front side of the resulting film support was coated with a gelatinsilver halide emulsion as commonly used for microfilm purposes.

Thanks to the polymer beads in the undermost layer the antihalationcomposition at the rear side of the film support in rolled up conditionappeared to withstand storage at high relative humidity verysuccessfully.

During development of the microfilm material the antihalation layerdissolved completely and discoloured spontaneously so that nothingremained at the rear side of the film support.

EXAMPLE 7

A gelatin silver bromoiodide (2 mol % of iodide) X-ray emulsioncomprising per kg 80 g of gelatin and an amount of silver halidecorresponding to 190 g of silver nitrate was coated on both sides of asubbed cellulose triacetate support at a ratio of about 27 sq.m (perside of support) per kg of emulsion.

At both sides of the support, the emulsion layers while still wet werecovered with a gelatinous protective layer or antistress layer from thefollowing coating composition:

gelatin: 30 g

sodium diisooctyl sulphosuccinate (5% aqueous solution): 28 ml

antistatic agent: 2 g

formol (4% aqueous solution): 30 ml

matting agent: 28.8 g

water to make: 1000 ml

10% aqueous solution of ethoxylated ricinoleic acid containing 40% ofethylene oxide groups was used as antistatic agent.

As matting agent polymer beads were used, which had been preparedaccording to preparation example 1, the average size of the polymerbeads being 2.2 μm. The gelatin antistress layers were coated at a ratioof 1.1 g of gelatin per sq.m and had a thickness of 1.1 μm each. Thephotographic element obtained was compared with a material prepared inan analogous way but comprising as matting agent a dispersion ofpolymethyl methacrylate particles prepared by dispersing in water asolution of the polymer in ethyl acetate.

The latter material, unlike the photographic element comprising thepolymer beads prepared as described in preparation example 1, had anundesirable milky appearance.

The antistatic properties of the photographic material comprising theprotruding polymer beads as described in preparation example 1 were alsoinvestigated. They were determined on the one hand by measuring thetriboelectric charging of the photographic X-ray film element by rubbingagainst rubber, packaging of interleave paper and brass and on the otherhand by estimating the discharge images produced in the emulsion layerby the sparks formed on rubbing the material in the dark against rubber,brass, polyvinyl chloride and intensifying lead screens, whereupon thelight-sensitive element was developed to make visible the dischargeimages produced. In both cases the antistatic properties proved to beexcellent.

The polymer beads of preparation 1 in the gelatin antistress layers didnot have any deleterious or adverse influence on the silver halide X-rayemulsion layer.

EXAMPLE 8

2.25 g of bis-1,5-(3-methyl-2-pyrazolin-5-one)-pentamethine oxonol weremixed with 250 g of standard sand 20/30 (designation G-190 by AmericanSociety for Testing Materials), the sand being furnished by OttawaSilica Company Ottawa, Ill., USA, 0.225 g of sodium salt ofoleylmethyltauride and a small amount of water to form a highly viscouspaste. This mixture was ground for 4 h in a sand mill and then filteredthrough a glass filter. The sand on the filter was washed with water toremove the adsorbed dye from the sand particles.

A volume of 450 ml of this filtrate was added to a solution of 75 g ofgelatin of 975 ml of water with a temperature of 36° C.

The resulting gelatin dispersion was admixed with a dispersion inwater/ethanol of polystyrene beads described in preparation 2 and withthe necessary coating aids and applied to the rear side of a biaxiallyoriented polyethylene terephthalate film support such that 0.150 g ofbis-1,5-(3-methyl-2-pyrazolin-5-one)-pentamethine oxonol and 5 g ofgelatin were present per sq.m.

The front side of the subbed film support was coated with a cadmium-freelithographic silver halide emulsion consisting of 76 mol % of silverchloride, 23 mol % of silver bromide, and 1 mol % of silver iodide,which had been spectrally sensitized with1-acetylmethoxycarbonylmethyl-substituted,1-hydroxyethoxycarbonylmethyl-substituted, or1-ethoxycarbonylmethyl-substituted5-[(3-sulphobutyl-2-benzoxazolinylidene)-ethylidene]-3-phenyl-2-thiohydantoin.The emulsion containing 0.6 mol of silver halide per kg and 0.1 millimolof spectral sensitizer per mol of silver halide was coated in a ratio of0.07 mol of silver halide per sq.m of film support. During developmentof the lithographic material the antihalation layer discolouredcompletely. Residual stain resulting either from the antihalation dye orfrom the sensitizing dyes used in the emulsion was practicallyinexisting.

Thanks to the polymer beads in the remaining discoloured antihalationlayer the sticking tendency of the photographic element against othersurfaces was reduced very efficiently. The formation of Newton's ringsand the generation of static electricity were avoided also.

EXAMPLE 9

Two different amounts of dried non-ground polymer beads prepared asdescribed in preparation 1 were added to two equal amounts ofpolyethylene terephthalate granules. After mixing, extrusion, andchilling amorphous plates were obtained. Samples were taken from theseplates and stretched 3.5 times biaxially in such a way that orientedpolyethylene terephthalate film supports were obtained. Next, thesamples were heat-set for 10 min at 220° C.

It was found that the addition of a small amount of polymer beads (0.05%by weight of beads calculated on the total weight of polymer beads andpolyethylene terephthalate granules) caused the static friction peaks,which are related to the disturbing sticking, blocking and difficultwinding behavior, to drop by a factor of more than 10 and made thedynamic friction coefficient decrease markedly as well.

An addition of 0.1% by weight of polymer beads resulted in even lowerfriction coefficients, but the transparency to light of the film supportwas reduced slightly, so that the film support could not be used anymorefor certain purposes. After the heat-setting step no staticfriction--and as a result thereof no stick-slip--could be observedanymore.

We claim:
 1. Photographic element comprising a support and at least onesilver halide emulsion layer wherein said element comprises in a layerof said photographic element and/or in the support thereof finelydivided solid spherical polymer beads obtained by the steps of:(A)dissolving in an aqueous solvent mixture of water and at least onewater-miscible polar organic solvent:(1) at least one α,β-ethylenicallyunsaturated monomer capable of forming a polymer that is soluble in themonomer(s) present in said aqueous solvent mixture but which isinsoluble in said aqueous solvent mixture, (2) a free radical-formingpolymerization initiator that is soluble in the aqueous solvent mixture,and (3) a graft-polymerizable polymer containing hydrophilic groups, andcapable of forming a graft polymer that remains soluble in said aqueoussolvent mixture, the weight ratio of said graft-polymerizable polymer tosaid monomer(s) being in the range from 1.5:100 to 8:100 and a weightratio of polymerization initiator to monomer(s) from 0.1:100 to 5:100,and (B) heating the solution obtained to a temperature from 50° C. tothe reflux temperature thereof with continuous stirring to initiate bypolymerization the simultaneous massive formation of homopolymer orcopolymer from said monomer and precipitation thereof, and the formationof a small porportion of graft polymer,said beads characterized in thatthey comprise a nucleus and an envelope which have differentcompositions, said nucleus being made up primarily of polymerizedα,β-ethylenically unsaturated monomer(s) and said envelop being made upprimarily of grafted polymer chains formed from said polymer containinghydrophilic groups and said α,β-ethylenically unsaturated monomer(s). 2.Photographic element according to claim 1, wherein said polymer beadsare present in an antihalation layer comprising an antihalation dye orpigment.
 3. Photographic element according to claim 2, wherein saidantihalation dye or pigment is carbon black.
 4. Photographic elementaccording to claim 1, wherein said beads are present in a polyethyleneterephthalate support.
 5. The photographic element of claim 1 whereinsaid α,β-ethylenically unsaturated monomer is methyl methacrylate orstyrene.
 6. The photographic element of claim 1 wherein saidgraft-polymerizable polymer is co(styrene/maleic acid monosodium salt)(50/50).
 7. The photographic element of claim 1 wherein saidα,β-ethylenically unsaturated monomer is methyl methacrylate; saidgraft-polymerizable polymer is co(styrene/maleic acid monosodium salt)(50/50); the aqueous solvent mixture is a mixture of equal volumes ofwater and ethanol, and the polymerization initiator is potassiumpersulphate.